Three different approaches are commonly used to form the outermost surface layer of a bituminous membrane roofing system, i.e. a roofing system composed of one or more layers of a water-proof membrane commonly made from a bituminous material or analog.
In the first, referred to here as the “in-situ aggregate” approach, a layer of bitumen (asphalt) or other adhesive is laid down on the underlayment of the roofing system, i.e. the portion of the roofing system under the outermost layer, and a layer of aggregate typically on the order of about ⅜ inch (˜1 cm) in diameter is laid down on and partially embedded in the adhesive.
In the second, referred to here as the “liquid applied membrane” approach, the outermost surface layer is formed by coating the underlayment with a roof paint or covering which forms a relatively thick (i.e. membrane-like) waterproof coating.
In the third, referred to here as the “cap-sheet” approach, the outermost surface layer is formed from multiple preformed membranes or sheets (hereinafter “cap-sheets”) which are laid up or arranged in an adjacent, overlapping edge fashion (like shingles in a shingle roof) to cover the entire roof surface to be covered. A layer of bitumen (asphalt) or other adhesive, typically applied to one or more lateral edges of each cap-sheet, is used to secure these cap-sheets to one another as well as the underlayment. Typically, these cap-sheets are made from a fibrous web or sheet of fiberglass, polyester fiber or both impregnated with bitumen (asphalt) or other suitable adhesive. Not infrequently, these cap-sheets are also covered with a suitable aggregate during manufacture for adding desired texture and/or color. Because the edges of adjacent cap-sheets are normally adhered atop one another, such aggregate is normally much smaller in size, typically on the order of ⅛ inch (˜0.3 cm) in diameter or less.
Commonly assigned application Ser. No. 11/250,075, filed Oct. 13, 2005, the disclosure of which is incorporated herein by reference, describes a “cool roof” covering, i.e. a roofing system having high solar reflectance and high heat emittance, in which the outermost surface layer of the bituminous membrane roofing system is formed using the in-situ aggregate approach. For this purpose, light-colored aggregates are preferably used since light-colored materials absorb less infrared radiation than darker materials and hence have better solar reflectance and thermal emittance properties. Depending on the particular aggregate and adhesive used, the cool roof covering disclosed there can have a solar reflectance of as much as 90% and a thermal emittance of as much as 95%.